The present invention relates to an inflatable contractable tension actuator inflatable in response to increasing fluid pressure.
Earlier inflatable, contractable actuators designed for providing a selected tension force between two points include U.S. Pat. No. 2,483,088 issued Sept. 27th, 1949 to de Haven which is composed of an inner elastomeric tube and an outer tensioning tube composed of strands interwoven on the diagonal, forming a plurality of left-handed and right-handed helices in the shape of a continuous tube. Radially directed force on the helically wound strands is provided by the inner tube in response to increasing the fluid pressure therein. Expansion of the helices translates into overall contraction and resultant tension applied to the actuator end supports.
U.S. Pat. No. 2,844,126 issued July 22, 1958 to Gaylord discloses an elongated expansible bladder made of flexible elastomeric material surrounded by a woven sheath forming an expansible chamber which contracts in length when expanded circumferentially by pressurized fluid. The sheath and end connectors translate radial expansion to axial force on a load.
U.S. Pat. No. 3,645,173 issued Feb. 29th, 1972 to Yarlott discloses an elongated flexible thin-walled bladder coupled at either end to coupling member end supports. The bladder expands or contracts radially in response to increased or decreased fluid pressure in the bladder, respectively, translating to axial movement of the end supports from extended or retracted positions, respectively. A network of spaced apart longitudinally-extending inextensible strands coupled by spaced apart inextensible strands embedded in the bladder, prevent elastic expansion of the shell and assist in translating radial force into axial tension.
Russian Patent No. 291,396 issued in 1971, discloses a flexible bladder with non-stretchable threads fitted in the tube walls and affixed to end terminals similar to Yarlott.
An important source of failure of devices such as de Haven arises from rubbing of the inextensible strands on the bladder. Such friction is at a maximum at the start of any contraction or extension due to the static nature of the friction and the requirement to break through this relatively high level of static friction before experiencing a lower dynamic friction. In devices such as de Haven, elastic hysteresis occurs due to expansion of the bladder surrounded by the strands.
The second limitation of some foregoing devices arises because of the relatively limited amount of contraction as a percentage of the uncontracted distance between the actuator ends that such actuators can achieve. The percentage contraction of the de Haven and Gaylord actuators is limited by the need to change the angle of the woven strands in the outer sheath during contraction. The amount of pulling force and the percentage contraction of an axially contractable actuator is directly related to the volumetric expansion of the bladder since the work done by the actuator equals the pressure therein multiplied by the total change in volume inside the actuator. In the above devices, the volume change inside the bladder is set substantially by the volume change inside the inextensible strands or cables, sometimes referred to as the spindle volume.
Although Yarlott, de Haven and Gaylord refer to a requirement for only flexible material for the bladder, de Haven and Gaylord indicate elastomeric material as being preferable. It has been discovered that operation of devices such as de Haven and Gaylord is enhanced by the lateral forces exerted on the strands as a result of elastic expansion of the bladder between the strands. Unfortunately, the high friction forces on, and tension forces in the fabric at these locations drastically increases the likelihood of actuator failure.